Wood and other cellulose based products exposed in an outdoor environment are biodegradable, primarily through attack by microorganisms and insects. As a result, they will decay, weaken in strength, and discolor. Microorganisms causing wood deterioration include brown rots such as Postia placenta, Gloeophyllum trabeum and Coniophora puteana, white rots such as Irpex lacteus and Trametes versicolor, dry rots such as Serpula lacrymans and Meruliporia incrassata and soft rots such as Cephalosporium, Acremonium, and Chaetomium. Insects which can destroy wood include termites, beetles, ants, bees, wasps, etc. Wood preservatives are used for preserving wood and other cellulose-based materials, such as paper, particleboard, textiles, rope, etc., against organisms responsible for the deterioration of wood.
Wood is also subject to destruction or degradation through other destructive agencies. In particular, wood is generally highly flammable, which can limit its usefulness in applications involving high temperatures or proximity of flames or flammable materials. Boron compounds have been widely used to provide wood with broad spectrum protection against most wood destroying organisms, such as fungi, termites, ants, bees, beetles, etc. Boron compounds, particularly inorganic compounds, are also known to have a flame retardant effect. An advantage of boron compounds over other wood preservatives and flame retardants is their low mammalian toxicity and minimal environmental impact.
Water-soluble boron compounds, such as boric acid, sodium borate and disodium octaborate tetrahydrate (DOT) have been widely used on wood based products for the past few decades. Several boron-containing wood preservative systems, such as copper-chromium-boron (CCB) and copper-boric acid-azole (CBA), have been developed. However, a major limitation of these commercially available preservatives is that the boron compounds readily leach from wood when exposed to environmental moisture, resulting in a diminished bioefficacy, especially over extended time periods. In an effort to reduce the high leachability of boron compounds from treated wood in a moisture environment, many efforts have been directed by researchers in the wood preservation area to fix the boron in the treated wood. U.S. Pat. No. 6,821,631 discloses a two step treatment process for preventing leaching of boron compounds from wood. Wood is treated with an alkali silicate solution followed by treatment with an alkali borate solution. It is thought that the silicate and borate forms a borate-silicate polymer within the wood substrate, imparting leach resistance to wood treated with boron compounds. However, due to the incompatibility of the silicate and borate solutions, this method is likely to create a cross contamination problem in commercial wood treating plants. In addition, the two-step treating process is expensive and time consuming.
U.S. Pat. No. 6,896,908 discloses a leach resistant borate preservative containing borate, copper or zinc, and organic acids.
Aqueous ammonia-based solutions have been widely used to solubilize metal borates, such as copper borate, zinc borate and calcium borate in an attempt to fix borates in wood. U.S. Pat. No. 5,207,823 describes a boron fixation method which includes dissolving copper borate or zinc borate in ammonia or other volatile agents. When the boron containing formulation is impregnated into wood, the volatile agents evaporate, leaving water-insoluble copper or zinc borate residue on the interstices of the wood. U.S. Pat. No. 2,194,827 discloses a boron wood preservative containing an aqueous ammoniacal solution of copper or zinc borate. Similarly U.S. Pat. No. 2,573,253 discloses a process for treating wood with a solution of copper borate in ammonia. However, current processes generally require the dissolution of the boron compound in a solvent, usually ammonia, which can be difficult and dangerous to work with due to its noxious fumes.
In addition to boron-containing biocides, existing wood preservatives can also contain other types of biocides, such as organic biocides. However, many organic biocides have limited water-solubility. Therefore, solubilizing agents, surfactants and wetting agents are often added to either solubilize or form emulsions of the organic biocide to formulate a product that is suitable for the treatment of wood or other cellulosic substrates.
However, the solubilizing agents, surfactants, and wetting agents are costly and the use of these products can result in enhanced leaching of the biocides when the treated material comes into contact with moisture, giving rise to field performance problems and environmental issues. Such enhanced leaching is considered to be the result of the solubilizing agents, surfactants and wetting agents which remain in the wood after treatment.
Despite many efforts to address these deficiencies in existing wood preservatives, there is an unmet need for aqueous boron-based wood preservatives which, upon contact with moisture, exhibit only minimal leaching into the environment.
A few inorganic boron compounds, such as for example, zinc borate, have been employed as flame retardants in wood applications. Generally, the compounds have been used by as a powder incorporated into a matrix. For example, the boron compound is incorporated into plastic by blending a polymer material with boron compounds. In an exemplary use of boron compounds in wood applications, zinc borate is mixed with wood particles, chips fibers or sawdust and a glue or adhesive matrix to form a wood composite product such as, for example Oriented Stand Board (OSB), or particle board. In another example, a boron compound-containing glue or adhesive matrix is used to join wood plys or veneers together (so-called “glue-line” use) to form plywood.
However, the above methods for incorporating flame retardant boron-containing compounds into wood or wood products is only applicable to composite wood products. In order that the boron compound be present throughout the volume of the wood product, it is necessary to mix it with adhesives and chips, pieces or particles of wood prior to the formation of the composite wood product.
A boron compound flame retardant composition, preferably free of polymer or glue matrices, and a method for applying the composition to a wood or wood product substrate, such that appreciable penetration is achieved, would be welcomed in the art.